logger: avoid data bursts by distributing slow subscription updates over time

There's an increasing amount of slow logged topics at 1-2Hz, which were all
updated in the same logger iteration, leading to data bursts. For log
streaming this started to exceed uart buffer sizes. By distributing updates
more equal over time those bursts are removed, reducing buffer size
requirements.

Tests showed during steady state a reduction of maximum topic updates per
iteration from 40 down to 17.
Also the SD log buffer fill level is more constant.

platforms/common/uORB/SubscriptionInterval.hpp

@@ -152,6 +152,11 @@ public:
 	 */
 	void		set_interval_ms(uint32_t interval) { _interval_us = interval * 1000; }
 
+	/**
+	 * Set the last data update
+	 * @param t should be in range [now, now - _interval_us]
+	 */
+	void		set_last_update(hrt_abstime t) { _last_update = t; }
 protected:
 
 	Subscription	_subscription;